Lecture 9: April 17, 2026

Reading: text, §8.4, 10.1–10.2.2
Assignments: Homework 2, due April 24, 2026

1. Greetings and felicitations!

2. Role-based access control

3. Cryptography
   1. Codes vs. ciphers
   2. Attacks: ciphertext only, known plaintext, chosen plaintext
   3. Types: substitution, transposition

4. Symmetric Cryptography
   1. Monoalphabetic (simple substitution): f(a) = a + k mod n
   2. Example: Caesar (shift) cipher with k = 3, RENAISSANCE → UHQDLVVDQFH
   3. Polyalphabetic: Vigenère, f_i(a) = a + k_i mod n
   4. Cryptanalysis: first do index of coincidence to see if it is monoalphabetic or polyalphabetic, then Kasiski method.
   5. Problem: eliminate periodicity of key
   6. Perfect secrecy: when the probability of computing the plaintext message is the same whether or not you have the ciphertext; only cipher with perfect secrecy: one-time pads; C = AZPR; is that DOIT or DONT?

Matt Bishop Office: 2209 Watershed Sciences Phone: +1 (530) 752-8060 Email: mabishop@ucdavis.edu

ECS 153A, Computer & Information Security & Privacy I Version of April 19, 2026 at 6:17PM

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